Wavelength division multiplexing, termed WDM, (discussed in, for example, Hill, British Telecom Technology Journal 6(3): 24-31) is a technique of considerable benefit in optimizing transmission of signals through fiber optic networks. In wavelength division multiplexing, traffic signals to be sent out by a station are modulated on to a number of carrier signals at different predetermined carrier wavelengths. Each predetermined carrier wavelength is allocated according to the identities of the send station and of the intended receive station. Predetermined carrier wavelengths will be spaced sufficiently far apart in wavelengths that they can be discriminated from each other by components of the fiber optic system, but in many networks will need to be grouped sufficiently closely that all carrier wavelengths can be amplified satisfactorily by the same amplifier in a repeater (or in unrepeated systems, to be carried long distances without significant loss). The carrying capacity of a single fiber is enhanced by WDM--rather than carrying a single signal, the fiber is simultaneously carrying several signals, each of a different wavelength.
Most such transmission networks have a number of nodes at which one or more branches form away from a main trunk or ring. Typically, at these nodes one or more carrier wavelengths are dropped down one fiber of the branch and one or more carrier wavelengths (which may be the same as, or different from, those dropped from the trunk or ring) are added to the trunk or ring from another fiber of the branch. The component which performs such a function is an Add/Drop Multiplexer (ADM).
WDM is particularly well adapted to efficient routing of signals between send and receive stations. As different signals have different carrier wavelengths, optical components can be used to route signals appropriately by directing them according to the carrier wavelength of the signal.
This can be done in an active manner, by splitting the signal into its component carrier wavelengths with a prism or similar component, and actively processing and routing the split signals to desired outputs. This solution is appropriate for use in an integrated device: a basic design for a multiplexer of this type is discussed in Dragone et al in IEEE Photonics Technology Letters 3(10): 896-899, and designs employing arrayed-waveguide gratings are disclosed for an ADM in Okamoto et al in Electronics Letters 31(9): 723-4 and for an optical splitter/router in Inoue et al in Electronic Letters 31(9): 726-7.
Alternatively, passive optical components can be used which respond differently to different carrier wavelengths. This enables passive network to be constructed.
The use of wavelength division multiplexing branching units (WDM BU) in optically amplified networks present new problems for the control and management of the networks. One of the problems is matching of the optical level of the added channel to the other channels passing along the trunk. I present a simple arrangement for controlling the power of the added channel from a branching unit to a trunk, so that it is added back into the system at the optimum level.
To equalize the performance of the different channels in a WDM system, pre-emphasis is applied to the powers of the channels. This may be effected in the manner described in A. R. Chraplyvy, J. A. Nagel, R. W. Tkach, "Equalisation in Amplified WDM Lightwave Transmission Systems", IEEE Photonics Tech. Lett., Vol. 4, pp. 920-922, 1992. This results in a system dependent power profile for the different channels. The dropping of channels from the line, and subsequent adding in of new channels (at the same wavelengths or even at different wavelengths) requires ideally that the level of the added channel(s) is such that it matches the trunk channels which pass straight through the BU.
Various methods of adjusting the levels of optical signals on a trunk are as follows:
European Patent Application Number 0663738A1 discloses an arrangement by which the amplitude of all signals in the trunk of an optical transmission system are controlled as a function of wavelength in a "regulation band" in the transmitted wavelength spectrum. It does not relate to a branching unit and provides no teaching as to how to adjust the level of a carrier signal to be added to a trunk from a branch unit in dependence upon the level of the carrier wavelength dropped.
European Patent Application No. 0543570A3 relates to the selective adjustment of the optical powers of each of the channels in an optical transmission system to equalise their powers by the provision of a telemetry signal from a station at the remote end of a trunk which relates to the level of that signal at the remote end of the trunk to a station at the input to the trunk. This does not relate to a wavelength division multiplexing add/drop branching unit and again provides no teaching of how to provide an add signal with different traffic signals back onto the trunk.
Japanese Patent Publication Number JP5110511 describes an arrangement by which a constant amplitude of two wavelength can be produced on a trunk at the output of an amplifier by comparing the generated wavelengths each with a different reference value so as to inject a compensating signal at a different one of a pair of multiplexers. It is not concerned with dropping and adding wavelengths between the trunk and a spur terminal.
The disclosure of Japanese Patent Publication Number JP5063643 is difficult to understand but seems to relate to the adjustment of gain of an optical amplifier 11 in dependence upon light sensed at the output of a trunk. There is no suggestion of dropping traffic signals from a trunk to a spur and adding traffic signals from such a spur back onto a trunk.